/Engine/Source/Runtime/GeometryFramework/Public/Components/DynamicMeshComponent.h
You must use this function if the mesh triangulation has been modified, or if polygroups or material assignments have been changed, or if Normal/UV/Color topology has changed (ie new split-vertices have been introduced). If only vertex attribute values (position, normals, UVs, colors) have been modified, then Notify Vertex Attributes Updated can be used to do a faster update.
virtual void NotifyMeshModified()